1. Technical Field
The present invention relates to a thin film solar battery having a substrate made of glass. flexible plastic stainless steel or the like, on which electrodes and a photoelectric conversion layer are laminated, and a method and apparatus for producing the thin film solar battery.
2. Background Information
A thin film solar battery is produced using a glass substrate, or a flexible plastic or stainless steel film having a thickness of about 100 .mu.m as a substrate. The thin film solar battery is so constituted that a photoreflective electrode, a photoelectric conversion layer and a transparent electrode are laminated on a primary surface of the substrate. The thin film solar battery is also constituted, for example, such that the transparent electrode is formed on the primary surface of the substrate, and the photoelectric conversion layer and the photoreflective electrode are formed on the transparent electrode, or such that another transparent electrode is used instead of the photoreflective electrode so that the photoelectric conversion electrode is interposed between these two transparent electrodes.
FIG. 1 is one example of a cross-sectional structural view of a conventional thin film solar battery. A photoreflective electrode 102 formed on a substrate 101 is made of a metal having a high reflectance such as Al (aluminum) or Ag (silver). The thickness of the photoreflective electrode 102 is usually about 0.1 to 1 .mu.m.
These metals relatively readily react mutually with Si (silicon) which is a component of a photoelectric conversion layer 103 formed on the photoreflective electrode 102 so that the former is alloyed with the latter.
To prevent such a reaction, a metal such as Cr (chromium), Ni (nickel), Ti (titanium) or stainless steel, or a metal oxide such as ZnO (zinc oxide) is formed thinly as a diffusion blocking layer (barrier layer) on the metal having a high photo reflectance, to form a photoreflective electrode (not shown). The thickness of this diffusion blocking layer is normally about 0.1 .mu.m or less.
The photoelectric conversion layer 103 is formed by a PIN junction of non-single crystalline semiconductors that mainly contain silicon. The photoelectric conversion layer 103 has a single cell structure using one PIN junction, a two-layer tandem cell structure having two PIN junctions connected in series, a three-layer tandem cell structure having three PIN junctions connected in series, etc. These structures are appropriately selected in accordance with the application or purpose.
As a method of producing the PIN junction of the nonsingle crystalline semiconductors that mainly contain silicon, a glow discharge decomposition method (plasma-enhanced CVD) is used. The glow discharge decomposition is a method wherein a reaction gas resulting from mixing silane gas, used as a source of silicon, with a dilution gas such as hydrogen, as occasion demands, and a doping gas such as diborane or phosphine, for the purpose of controlling valence electrons, is introduced into a reaction chamber which is under vacuum. Glow discharge is developed in the reaction chamber. The reaction gas is decomposed by the discharge energy, and the decomposed reaction gas is deposited on the substrate to form a film.
In this method, the non-single crystalline semiconductor is produced by applying a high frequency electric power (normally 13.50 MHz) of 0.01 to 10 mW per unit area (cm.sup.2) under conditions where the substrate temperature is 100 to 250.degree. C., preferably 150 to 240.degree. C., and the reaction pressure is 0.01 to 10 Torr, preferably 0.03 to 1 Torr.
It is known that non-single crystalline semiconductor films produced by the above glow discharge decomposition contain hydrogen in an amount of several to several tens of atom per cent therein, depending on the manufacturing conditions. The content of hydrogen can be intentionally changed depending upon the substrate temperature when forming a film, the mixture ratio of reaction gases (silane, hydrogen), and discharge power.
For an N-type non-single crystalline semiconductor, there is used a silicon thin film to which phosphorus (atomic symbol: P) is added, for example, a (amorphous)--Si:H, pc (microcrystal)--Si:H, and a-SiC:H to which carbon is added. For a substantially-intrinsic I-type non-single crystalline semiconductor, there is used a-Si:H, a-SiGe:H to which germanium is added, and a-SiC:H. For a P-type non-single crystalline semiconductor, there is used a-Si:H, .mu.c-Si:H, and a-SiC:H wherein boron (atomic symbol: B) is added to each.
In the photoelectric conversion layer, the N-type layer is 5 to 50 nm in thickness, preferably 20 to 30 nm, the I-type layer is 30 to 1000 nm, preferably 30 to 60 nm in thickness, and the P-type layer is 5 to 250 nm, preferably 10 to 50 nm in thickness. In the photoelectric conversion layer, the N-type layer, the I-type layer, and the P-type layer are generally laminated on the photoreflective electrode in the stated order. Layers in order of P, I and N may also be laminated.
The transparent electrode 104 is made of an alloy of indium oxide and tin, tin oxide or the like, and is formed by sputtering. The thickness of the transparent electrode layer is 40 to 200 nm.
An apparatus for producing the photoelectric conversion layer includes a reaction chamber, a gas exhaust unit for exhausting gas in the reaction chamber to evacuate the interior of the reaction chamber, a gas introduction unit for introducing a reaction gas into the reaction chamber, a substrate holding unit for holding a substrate on which a film is formed in the interior of the reaction chamber, and a glow discharge generating unit.
The glow discharge generation unit includes two parallel electrodes consisting of a cathode and an anode, and is generally called the capacitive coupling type. There are many cases in which the anode is held at ground potential and serves as the substrate holding unit and substrate heating unit. The cathode is connected to a high-frequency power source, and a high-frequency power is applied between the cathode and the anode to generate a glow discharge.
The photoelectric conversion layer of the thin film solar battery is formed by the PIN junction, and the reaction gas for forming each of those layers is different from each other. For that reason, the apparatus for producing the photoelectric conversion layer is equipped with an exclusive reaction chamber for each of the P-, I-, and N-layers.
In producing the thin film solar battery using a flexible substrate, there is used an apparatus in which the flexible substrate wound in the form of a roll is unwound from one roll, a film is formed on the flexible substrate while the flexible substrate is allowed to pass through the reaction chamber, and the flexible substrate is wound up on the other roll. This is called the roll-to-roll type.
FIG. 2 is one example of a semiconductor film producing apparatus of the conventional roll-to-roll type. This apparatus includes an unwind chamber 201 for unwinding a flexible substrate 200 wound on a bobbin 225; a wind-up chamber 205 having a wind-up bobbin 226; respective reaction chambers 202, 203, and 204 for P, I, and N layers; slits 221, 22,223, and 224 for partitioning the respective reaction chambers; substrate heating units 206, 207, and 208; glow discharge generating electrodes 209, 210, and 211 connected to high frequency power supplies 212, 213, and 214; gas exhaust units 215, 216, and 217; and gas introduction units 218, 219, and 220.
The thin film solar battery using the non-single crystalline semiconductor that mainly contains silicon suffers from a problem of light deterioration such that the photoelectric conversion characteristic is deteriorated by light irradiation. It has been confirmed that the light deterioration is caused by deterioration of the I-type layer of the photoelectric conversion layer, which is made of an alloy of substantially-intrinsic amorphous silicon hydride.
It is known that the light deterioration of the thin film solar battery is changed in accordance with external conditions such as light intensity or temperature at the time of light irradiation, or conditions under which the photoelectric conversion layer is manufactured.
It is also known that the deterioration of the I-type layer made of an alloy of amorphous silicon hydride is caused by an increase of the defect density in the I-type layer. The defect density can be measured by a method such as the electron spin resonance method or the like. The increase in the detect density due to light deterioration has also been observed through the electron spin resonance method.
Specifically, it is known that the initial state defect density of about 1.times.10.sup.15 per cm.sup.3 is increased to about 5.times.10.sup.16 or more due to light deterioration.
The reasons why the defect density increases have been variously discussed and have not yet been sufficiently proven. One of the reasons which has been indicated is that the increase in the defect density is influenced by (bonding) hydrogen contained in the film made of an alloy of amorphous silicon hydride. See e.g., Kazuo Morigaki,"Light inductive phenomenon in amorphous silicon hydrogen", Solid-State Physics, Vol. 29, No. 5, 1988, at p.1. This result alleges that the light deterioration is reduced as hydrogen contained in the film is decreased.
Although the density of (bonding) hydrogen in the amorphous silicon hydride alloy is changed in accordance with the conditions under which the film is formed, it is effective and necessary to increase the substrate temperature for the purpose of decreasing the hydrogen density.
In view of this, by comparing the light deterioration ratio of a solar battery having a larger hydrogen density in the I-type layer to the light deterioration of a solar battery having a relatively smaller hydrogen density, by changing the substrate temperature at the time of forming the I-type layer, it has been confirmed that the solar battery having the smaller hydrogen density is reduced in the light deterioration ratio more than the battery having the larger hydrogen density.
The solar battery having the larger hydrogen density has been manufactured at 120.degree. C. as the temperature at which the I-type layer is formed. The solar battery having the smaller hydrogen density has been manufactured at 240.degree. C. as the temperature at which the I-type layer is formed.
FIG. 3 shows the comparison result of the characteristics of the solar batteries each having a different hydrogen density of the I-type layer. Sample A has a hydrogen density of 28 atom percent, and sample B has a hydrogen density of 12 atom percent. The ratio of deterioration of these samples A and B after a light of AM1.5 and 100 mW/cm.sup.2 has been continuously irradiated for 1000 hours at a sample temperature of 50.degree. C. in an open circuit state has been compared. As a result, it has been observed that the ratio of deterioration in the sample A is about 40%, and the ratio of deterioration in the sample B is about 25%.
In comparing the initial photoelectric conversion characteristics of these solar batteries with each other, the initial efficiency (EFF) of conversion of the solar battery of sample A, having a high hydrogen density, is 9.5% whereas the efficiency of conversion of the solar battery of sample B is 7.5%. The difference in the initial conversion efficiency between the samples A and B is caused by the temperature at which the I-type layer is formed. The characteristics of these solar batteries are shown in Table 1. The difference in the conversion efficiency between these two samples appears in the fill factor (FF) and open-circuit voltage (VOC).
TABLE 1 ______________________________________ E.sub.ff (%) FF Jsc (mA) Voc (V) ______________________________________ Sample A 9.5 0.73 14.5 0.90 Sample B 7.5 0.61 14.8 0.86 ______________________________________
In Table 2, the optical gap (E.sub.g) of the I-type layer in sample A, which is high in the density of hydrogen, is 1.80 eV whereas the optical gap of the I-type layer in sample B, which is low in the density of hydrogen, is 1.72 eV. Other characteristics such as dark conductivity (6d), light conductivity (6p) and defect density (N.sub.d) do not have so large a difference between the samples A and B. Thus, the difference in open-circuit voltage between the samples A and B is caused mainly by the difference in the optical gap of the I-type layer between the samples A and B.
TABLE 2 ______________________________________ E.sub.g (eV) .sigma..sub.d (S/cm) .sigma..sub.p (S/cm) N.sub.d (cm.sup.-3) ______________________________________ Sample A 1.80 5 .times. 10.sup.-11 1.5 .times. 10.sup.-5 2 .times. 10.sup.15 Sample B 1.72 1 .times. 10.sup.-10 6 .times. 10.sup.-5 1.3 .times. 10.sup.15 ______________________________________
It is presumed that the difference in the fill factor is caused by the quality of the I-type layer or the junction interfaces between the P-type and N-type layers. In confirmation of this, when comparing the film characteristics of the respective I-type layers deposited on a quartz glass substrate under the same conditions between the samples A and B, a slight difference in the optical gap, the dark conductivity, and the light conductivity has been observed, but little difference in the defect density has been observed.
Hence, it is presumed that the difference in the fill factor is caused not by the film characteristic of the I-type layer per se, but by the interfaces on which the junctions of the I-type layer are formed.
The photoelectric conversion layer of the solar battery is formed by sequentially laminating, for example the N-type layer, the I-type layer, and the P-type layer in the stated order. What is influenced by the temperature at which the I-type layer is formed is an underlying layer of the photoelectric conversion layer. Hence, in this example, the interface between the N-type layer and the I-type layer causes the difference in fill factor between the samples A and B.
It has been shown that the initial characteristics of the solar battery are excellent when the temperature at which the I-type layer is formed is low and the density of hydrogen is high, but the light deterioration is excellent when that temperature is high and the density of hydrogen is low.
However, in the above results, it is impossible to produce a solar battery having a low light deterioration ratio without lowering the initial conversion efficiency.